The present invention relates to a method for controlling an engine and an automatic transmission of a vehicle and, more particularly, to a control method for engines and automatic transmissions that combines neutral idle control in the transmission with engine stop/start control.
To improve fuel economy, manufacturers have proposed shutting off a vehicle's engine while the vehicle is at rest, such as the vehicle being at rest at a stop light. Currently, if a control system shuts off the engine when the vehicle comes to a rest, the operator and occupants may experience noise, vibration, and harshness issues. These noise, vibration, and harshness issues are from abruptly ending the forward output torque (T′) of the vehicle's engine applied through the automatic transmission to the vehicle's wheels. Similar problems may also occur as the engine of the vehicle is restarted and the output torque (T′) is suddenly applied to the wheels. Another problem with using engine start/stop control strategies is that when the engine restarts in response to an engine start condition, such as the operator pressing the throttle pedal upon seeing a green light, the transmission may require time to build enough pressure (P′) against an actuator that controls the engagement state of an engagement element that transfers torque from the engine to the wheels. The delay in building pressure (P′) to engage the engagement element delays the forward motion of the vehicle, which is undesirable and presents safety concerns.
To address some of these problems, manufacturers have introduced a variety of methods to reduce the pressure in a transmission before stopping the engine. One such method requires the operator of the vehicle to shift the transmission into neutral, thereby releasing pressure (P) against the engagement device before the engine is shut off. Upon sensing that the gear selection is neutral, the control system stops the engine. To many consumers, the additional step of manually selecting neutral to improve fuel economy is unacceptable. This method is also subjected to delay associated with the building of pressure before the vehicle can accelerate after the operator moves the shift lever to drive.
To improve fuel economy, other manufacturers have used neutral idle control algorithms. Neutral idle control is used to reduce pressure in the transmission to disengage the torque transfer between the engine and the wheels when the vehicle comes to a stop, thereby saving fuel economy while the engine is running. Fuel economy is improved in neutral idle transmissions because the engine does not have the drag associated with providing forward torque, while the vehicle is stopped. Neutral idle control methods have not been used with stop/start engine control methods due to problems with this combination that include loss of pressure if the engine is stopped, and delay in building pressure after the engine is restarted. The output torque (T′), combustion torque (C′), and actuator pressure (P′) of an exemplary prior art transmission are represented in FIGS. 5–10.